The EUV Late-Phase: Statistical Results from 15 Years of Solar Dynamics Observatory Observations

TL;DR

This study provides the most comprehensive statistical analysis of the EUV late-phase (ELP) in solar flares over 15 years, revealing its occurrence rate, timing characteristics, and correlations with flare properties using SDO/AIA data.

Contribution

It offers new insights into the statistical behaviour, timing, and physical variability of the EUV late-phase in solar flares, based on a large dataset and advanced analysis methods.

Findings

ELP occurrence rate is about 9%, with no strong dependence on the solar cycle.

Typical ELP onset delay is 19 minutes, with a duration of 93 minutes.

Strong correlation between ELP rise/decay rates and flare impulsivity, but no direct phase correlation.

Abstract

Since its launch in 2010, the Solar Dynamics Observatory (SDO) has provided continuous, high-cadence, multi-wavelength observations of the Sun, capturing thousands of solar flares and offering new insights into coronal dynamics. Among the discoveries enabled by SDO is the EUV late-phase (ELP), characterised by a secondary enhancement in warm coronal emission occurring tens of minutes after the main flare. While recent work has demonstrated the relevance of the ELP for space weather, the statistical behaviours and physical origins are not fully understood. Here, we present the most comprehensive statistical analysis of the ELP to date, based on 15-years of Fe xvi (335 angstrom) observations from the Atmospheric Imaging Assembly onboard SDO (SDO/AIA). From a sample of 5335 isolated flares between 2010 and 2025, we identify and validate 467 ELP events. The overall ELP occurrence rate was found to be 9 percent, with no significant dependence on the solar cycle and only a modest enhancement in the low-mid M-class range. The ELP typically exhibited an onset delay of 19 minutes, a peak-to-peak delay of 88 minutes, and a duration of 93 minutes. Strong correlations were found between ELP rise and decay rates (p = 0.76), and between flare and ELP impulsivity (p = 0.61). However, a comprehensive pairwise analysis revealed no significant correlation between the flare and ELP phases. A Principal Component Analysis of flare and ELP properties identified several semi-independent axes of variability, broadly associated with late-phase temporal scale, impulsive heating characteristics, and the relative prominence of flare and late-phase intensity measures. These results highlight the continuing importance of SDO's long-term, high-resolution observations for uncovering new aspects of solar flare evolution and improving understanding of the Sun-Earth connection.